The documents WO 99/10712 and WO 02/50522 describe a method and apparatus for measuring fluid mixture (oil effluent made of water, oil and gas) flow rates. The apparatus comprises a Venturi, a pressure sensor comprising pressure takeoffs opening out into a measuring section of the Venturi, a temperature sensor, a source and a detector of gamma rays positioned on opposite sides of a throat of the Venturi. The source and the detector enable determining the density and the composition of the fluid mixture. The detector measures the attenuation of the gamma rays having passed through the fluid mixture in two different energy levels. The energy levels are such that the high energy count rate is essentially sensitive to the density of the fluid mixture, and the low energy count rate is also sensitive to the composition of the fluid mixture. The measurements of pressure and temperature enable determining the gas density and the gas flow rate under flow rate condition and normal conditions of pressure and temperature, respectively. The apparatus enables measuring the total mass flow rate, the water/liquid mass ratio, and the oil, water and gas flow rates. There is a risk of significant variation of the low energy water mass attenuation coefficients with the presence of salt species (e.g. NaCl, KCl, etc. . . . ) or other heavy atoms (e.g. H2S, CO2, etc. . . . ), as the concentration of salt species and heavy atoms may vary during the measurements. This variation in compositional elements has an impact, in particular on the water-liquid ratio measurement accuracy as measured with the prior art apparatuses and methods.
For example, the low energy mass attenuation coefficient for pure water is 0.03334 m2/kg and for water with 25% NaCl by mass is 0.05474 m2/kg, thus corresponding to a difference of around 0.02000 m2/kg. Further, in order to achieve an absolute error on the water liquid ratio inferior to ±0.05, it is necessary to know the low energy water mass attenuation coefficient with an accuracy better than ±0.00015 m2/kg. In applications where the water salinity may change with time, the low energy water mass attenuation coefficient needs to be monitored and updated. However, in the example of permanent sub sea applications, accuracy is difficult to maintain because liquids sampling is not regularly feasible and the measuring apparatus is not accessible for long periods of time.